An internal combustion engine is commonly classified as a spark-ignition engine, a compression-ignition engine or a continuous-flow internal combustion engine. A typical spark-ignition engine is shown in FIG. 11.
The combustion in a conventional spark-ignition engine is as follows an admixture supplied from outside into the cylinder is compressed by the upward motion of the piston in a combustion chamber when the portion of this compressed admixture closest to the spark plug is ignited by the spark plug, a flame front propagates, like a ring in water, from the ignition point throughout the combustion chamber. Finally all of the admixture in the combustion chamber is burnt.
As described above, there are two prime phases, i.e. before-combustion admixture and after-combustion gas, which are divided clearly by the flame front, the flame front propagating like a ring in water from a particular portion. Such flame propagation is a characteristic of a spark-ignition engine.
Spark-ignition engines include not only the above mentioned means but also the type wherein there is direct fuel injection to the combustion chamber, as shown in FIG. 12, and the CVCC type shown in FIG. 13. The difference between those two types and the above mentioned type is as follows. The former produces an admixture in the combustion chamber by direct fuel supply to the combustion chamber from a fuel injection-valve, while in the latter type, a substantially homogeneous lean admixture is supplied to the combustion chamber and a rich admixture is supplied to a sub-combustion chamber disposed in the cylinder head. When the spark plug ignites rich admixture in the sub-combustion chamber, the flame and the combustion gas belch out from the sub-combustion chamber to the combustion chamber in order to combust the lean admixture in the combustion chamber. However, the process of flame front propagating from a particular portion and spreading combustion to all the admixture in the combustion chamber are the same as the first mentioned conventional type.
Conventional compression-ignition engines are classified in three types, i.e. direct-injection, pre-combustion-chamber and swirl-chamber types, as shown in FIGS. 14 to 16 respectively.
In the direct-injection type, where the fuel is injected directly into the combustion chamber from an injection-valve, the fuel self-ignites and burns by contact with the air in the combustion chamber which becomes hot and highly compressed by the upward motion of the piston.
The pre-combustion-chamber type has a main combustion chamber and a pre-combustion chamber of small volume which is connected with the main combustion chamber through a small diameter connecting-hole. First, a part of the fuel injected from the injection-valve to the pre-combustion chamber is burn. Then as the pressure increases in the pre-combustion-chamber, the fuel together with the combustion gas is ejected through the connecting-hole into the main combustion chamber and burns all the fuel therein.
The swirl-chamber type provides a nearly spherical swirl-chamber in the cylinder head, the fuel supplied by the injection valve into the swirl chamber is mixed rapidly with the air by the effects of the swirl and for the most part is burnt therein. High temperature gas is then ejected through the connecting-hole into the main combustion chamber and completes the combustion.
In recent years, an increase in power output, a decrease in the pollutants in exhaust gas, a decrease in the fuel consumption, and a decrease in noise are the most important factors in the design of internal combustion engines, and these are all related to the combustion systems.
However, the combustion systems explained above have the following problems.
Since a combustion principle in the spark-ignition engine depends on the flame propagation as explained, if the fuel content in the admixture is low, the admixture is not ignited or some uncombusted part remains. On the other hand, if the fuel content is high, not only do the pollutants in the exhaust gas, i.e. HC and CO content, increase, but the NOx content increases due to the high combustion temperature. Moreover, fuel consumption also increases. In order to solve these problems the above mentioned direct-fuel-injection type was attempted, but it has a problem of mixing the fuel with the air in the combustion chamber and hardly any practical use of it can be seen today.
Another problem of the combustion system depending on the flame propagation is slow combustion speed. Increasing speed by swirl in the cylinder is recommended but the thermal loss at the wall of the cylinder increases and creates another problem of reduction of thermal efficiency of the engine. In order to improve the thermal efficiency it is necessary to increase a cycle efficiency by using a higher compression ratio thereby completing the combustion as soon as possible after the upper dead point and increasing the constant-volume-ratio. However, high compression ratios generate knocking.
A spark plug, shown in FIG. 17, is an indispensable component for the spark-ignition engine. Some problems with existing spark plugs are ignition difficulty due to a lean admixture or due to turbulence intended to increase combustion speed, quenching effects by the electrode, insulation failure as a result of carbon deposit or fouling and wear of electrodes, etc. , all of which form the basis for important requirements for the improvement of a combustion system in the spark-ignition engine. Therefore, various ideas for improvement of the spark plug have been proposed, for example in U.S. Pat. Nos. 1,929,748, 3,056,899 and 3,515,925.
A spark plug (A), from the specification of U.S. Pat. No. 1,929,748, is indicated in FIG. 18. Regarding spark plug (A), the combustion flame is ejected to the combustion chamber mainly in the direction of the axis of the spark plug (A) and the remaining combustion flame is ejected from passages spirally and causing a swirl, thereby promoting rapid and complete combustion of the admixture and providing an improved self cleaning effect on the spark plug itself.
Spark plug (B), from the specification of U.S. Pat. No. 3,056,899, is indicated in FIG. 19. The ignition of admixture takes place by generating a spark between the central electrode and an adapter. The combustion flame generated by the spark is ejected from a tapered cup to the combustion chamber, thereby ensuring immediate combustion of the admixture. Further, a pre-combustion admixture is brought inside the spark plug (B) through vents by negative pressure generated by ejection in order to prevent overheating and fouling around the central electrode.
Spark plug (C), from the specification of U.S. Pat. No. 3,515,925, is indicated in FIG. 20. In the spark plug (C), the spark is generated between the extremity of a center electrode and near a conical aperture of an outer electrode so that the combustion flame is mainly ejected outward where resistance is less. Thereafter, a residual combustion flame induces combustion in the admixture inside the spark plug (C) and ejects the combustion gases horizontally from the radial bores, i. e. the ejection of the combustion flame from both the conical aperture and the radial bores is designed for immediate combustion of the admixture in the combustion chamber.
In all the spark plugs mentioned above, a combustion flame is ejected mainly in the direction of the axis of the spark plug for accelerating the combustion. Thus, the combustion pattern is similar to the conventional flame propagation from a spark plug igniting point at the center, and so not much improvement is made in terms of reduction of combustion time, uniform combustion or complete combustion of all of the admixture.
The problems of compression-ignition engines are as follows:
With regard to the direct-injection type, a long ignition-delay and a high combustion apex pressure bring about large combustion noise, and thus a strong construction is necessary for the main frame of engine. Also a large volume of N0x and dark smoke is emitted in the exhaust gas and diesel knock tend to be caused. If swirl is generated in the combustion chamber in order to solve the above mentioned problems, a new problem of increase of thermal loss arises.
With regard to the pre-combustion chamber type, the surface area of combustion chamber is large because a main combustion chamber and a pre-combustion chamber are both provided. Also high temperature gas passing through a small diameter connecting-hole increases the thermal loss and the energy loss induced at said connecting -hole, thus idle-knock tends to occur at low r. p. m. and friction loss increases at high r. p. m. Further, two combustion chambers, i. e. the main combustion chamber and the pre-combustion chamber, make the engine construction complicated and expensive.
With regard to the swirl-chamber type, thermal loss is increased by the swirl in the swirl chamber, where the surface area of the swirl-chamber is large, since the formation of the admixture depends on the swirl, the torque reduces with reduced r. p. m. Further, it must have both the combustion chamber and swirl chamber, thus the construction is complicated and expensive.
It is obvious at present that, regardless of whether considering a spark-ignition engine or a compression-ignition engine, any improvements in the internal combustion engine which are designed to improve the engine efficiency and reduction of pollutants and costs, depend on further acceleration of the combustion speed and on complete combustion of the fuel. In the continuous-flow internal combustion engine, stable and complete combustion, reduction of pollutants in the exhaust gas and improvement of the combustion efficiency are also to be desired.